1. FIELD OF THE INVENTION
The field of the invention is reduction of coking in high temperature transfer lines, such as the transfer line from an FCC reactor to the FCC main column.
2. DESCRIPTION OF RELATED ART
Catalytic cracking is the backbone of many refineries. It converts heavy feeds into lighter products by catalytically cracking large molecules into smaller molecules. Catalytic cracking operates at low pressures, without hydrogen addition, in contrast to hydrocracking, which operates at high hydrogen partial pressures. Catalytic cracking is inherently safe as it operates with very little oil actually in inventory during the cracking process.
There are two main variants of the catalytic cracking process: moving bed and the far more popular and efficient fluidized bed process.
In the fluidized catalytic cracking (FCC) process, catalyst, having a particle size and color resembling table salt and pepper, circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts a source of hot, regenerated catalyst. The hot catalyst vaporizes and cracks the feed at 425.degree. C.-600.degree. C., usually 460.degree. C.-560.degree. C. The cracking reaction deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and the stripped catalyst is then regenerated. The catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air. Decoking restores catalyst activity and simultaneously heats the catalyst to, e.g., 500.degree. C.-900.degree. C., usually 600.degree. C.-750.degree. C. This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Catalytic cracking is endothermic, it consumes heat. The heat for cracking is supplied at first by the hot regenerated catalyst from the regenerator. Ultimately, it is the feed which supplies the heat needed to crack the feed. Some of the feed deposits as coke on the catalyst, and the burning of this coke generates heat in the regenerator, which is recycled to the reactor in the form of hot catalyst.
Catalytic cracking has undergone progressive development since the 40s. The trend of development of the fluid catalytic cracking (FCC) process has been to all riser cracking and use of zeolite catalysts.
Riser cracking gives higher yields of valuable products than dense bed cracking. Most FCC units now use all riser cracking, with hydrocarbon residence times in the riser of less than 10 seconds, and even less than 5 seconds.
Zeolite-containing catalysts having high activity and selectivity are now used in most FCC units. These catalysts work best when coke on the catalyst after regeneration is less than 0.1 wt %, and preferably less than 0.05 wt %.
To regenerate FCC catalysts to these low residual carbon levels, and to burn CO completely to CO2 within the regenerator (to conserve heat and minimize air pollution) many FCC operators add a CO combustion promoter metal to the catalyst or to the regenerator.
U.S. Pat. Nos. 4,072,600 and 4,093,535, which are incorporated by reference, teach use of combustion-promoting metals such as Pt, Pd, Ir, Rh, Os, Ru and Re in cracking catalysts in concentrations of 0.01 to 50 ppm, based on total catalyst inventory.
As the process and catalyst improved, refiners attempted to use the process to upgrade a wider range of feedstocks, in particular, feedstocks that were heavier, and also contained more metals and sulfur than had previously been permitted in the feed to a fluid catalytic cracking unit.
Refiners have tended to push their FCC units as much as possible, both with a view to maximizing yields of gasoline and light olefins and to process ever heavier feedstocks. Higher riser top temperatures increase yields of gasoline and light olefins, and also may improve somewhat the ability of the FCC unit to crack heavier feeds. Unfortunately, the heavier feeds, and/or the higher riser top temperatures, have produced reactor effluents having a temperature, and sometimes containing reactive materials which tend to form coke.
Coke formation in catalytic cracking units has been a problem since the beginning of cat cracking. Coke readily forms in any dead space. Dome coke, sometimes called the "fifth" kind of coke formed in FCC units is a severe problem in every FCC having a dome shaped vessel containing the cyclones and/or other equipment associated with the reactor outlet. The problem of dome coke was solved by adding small amounts of steam, typically 500 to 1000 #/hr, to purge the dome. Most FCC units now have this, but the practice of adding dome steam is so common that the reason for adding dome steam is rarely discussed.
Coking beneath the bubble cap trays in the fractionator associated with moving bed cracking units has also been a problem for almost 50 years. The high temperature vapor from the moving bed cracking unit would, if allowed to remain stagnant for a long time in the TCC main column, form coke inside the column. This problem was solved by adding copious amounts of quench liquid to the TCC column inlet, so that a two phase, quenched mixture is added to the main column.
With ever heavier feeds, and ever higher riser top temperatures, the transfer lines between the dome and main column are now starting to coke in some units. This is a severe problem, for several reasons.
As coke levels on transfer lines build, the coking tends to get worse, because the porous coke deposits provide an ideal place for fresh coke deposits to form. The reduced diameter of the transfer line increases pressure drop through the system, raising reactor pressures somewhat, which tends to adversely affect yields The coke deposition also increases the weight of the transfer line, which is usually designed to be full of hot vapor, rather than clogged with coke. In some units the problem of coking in transfer lines downstream of the FCC reactor has become so severe that the unit had to be shut down to permit replacement of the transfer line.
We studied a commercial FCC unit, which had a problem with coke deposition in the transfer line to the main column, and realized that the problem was caused by thermal formation of free radicals, which polymerized and laid down coke in the transfer line.
The conventional approaches used to solve coking problems in catalytic cracking units were not applicable. Although 500 or 1000 #/hr of dome steam does a good job of purging stagnant areas in the dome, it did nothing, so far as we could tell, toward reducing coking in transfer lines. The dome steam is minuscule compared to the amount and volume of hot product flowing through the transfer line. Although quenching the transfer line might seem to be applicable, we were concerned at the costs of this, and feared that it might make the problem worse, i.e., adding a liquid could deposit liquid on a hot surface and cause coke to form on the hot surface.
We discovered a way to reduce coking at essentially no capital expense, and with very little operating expense. Our solution required only that an effective amount of a coke suppressing additive be added, or present, in the transfer line from a cat cracker.